The present invention relates to a method for controlling output from an ultrasonic speaker and an ultrasonic speaker system capable of providing a difference frequency component (self-demodulated sound) having extremely strong directivity due to parametric array effect when emitting a signal as ultrasonic waves modulated by an audio signal from an ultrasonic transducer such as an ultrasonic vibration element.
An ultrasonic speaker is used for providing audio information to only a particular area or for other purposes since the ultrasonic speaker has extremely strong directivity compared with an ordinary speaker. For example, the ultrasonic speaker is equipped in an exhibition hall such as an art museum, thereat the ultrasonic speaker gives explanation about a work only to a person having approached the work and existing within a limited area near the work.
Since sound waves emitted from a speaker of an ordinary audio system are spherical waves, the characteristics of sound pressure level to frequency felt by a listener scarcely vary even when the listener listening the sound changes his/her angle with respect to the sound wave emitting surface of the speaker. However, in case of the ultrasonic speaker, sound waves generated from an ultrasonic transducer are plane waves. Thus, the characteristics of sound pressure level to frequency felt by the listener considerably vary when the listener listening the sound changes his/her angle with respect to the sound wave emitting axis of the ultrasonic transducer.
An example of this variation in the characteristics of sound pressure level to frequency is now discussed. It is assumed that the angle formed between an axis indicating the listener's front direction (hereinafter referred to as listener front direction axis) and the sound wave emitting axis of the ultrasonic transducer is 90 degrees, and that the left ear of the listener is positioned closer to the ultrasonic transducer than the right ear (this condition is hereinafter referred to as condition A). From this position, the listener gradually rotates anticlockwise until the angle mentioned above becomes 180 degrees (condition after 180° rotation is hereinafter referred to as condition C). Based on these assumptions, the self-demodulated sound entering the right ear of the listener is now examined. As the listener approaches the condition C in the anticlockwise direction from the condition A, the sound pressure level of the self-demodulated sound in the audio frequency band gradually increases. The increase rate of the sound pressure level is greater at a lower frequency. In this case, the characteristics of sound pressure level to frequency of the self-demodulated sound entering the left ear vary oppositely to the case of the right ear discussed above.
For improving the sound quality in the ordinary audio system, it is necessary to flat the characteristics of sound pressure level to frequency by filtering the audio signal or by other methods. However, in case of the ultrasonic speaker, unlike the case of the ordinary speaker, the characteristics of sound pressure level to frequency felt by the listener considerably vary when the listener listening the sound changes his/her angle with respect to the sound wave emitting axis of the ultrasonic transducer as explained above. Thus, when the ultrasonic speaker system is included in an application used by a listener whose listening position and angle are variable with respect to the sound wave emitting surface of the ultrasonic transducer according to possible conditions, the filter characteristics in filtering the audio signal are required to be varied in accordance with the current position and angle of the listener so that signal processing such as flatting the characteristics of frequency can be appropriately performed.
In order to meet this requirement, there is proposed an audio information providing method and a directivity-type audio information providing apparatus which can provide a sound space having high directivity, and further shift the sound space and vary information to be provided according to the position and movement of a target person (refer to Patent Document 1).
According to this audio information providing apparatus, the space range for providing audio information and the contents of the audio information to be provided are dynamically varied by using position determining means for determining the position of the target person and audio information output means for outputting audio information while tracing the target person based on the output from the position determining means.
[Patent Document 1] JP-A-2005-80227
This audio information providing apparatus provides the optimum sound space for the listener by controlling the angle of the sound wave emitting axis of the ultrasonic transducer according to the position of the listener and varying the frequency of carrier waves according to the distance from the listener. However, this related-art apparatus does not consider the angle of the listener with respect to the sound wave emitting axis of the ultrasonic transducer. Thus, there is a problem in that when the listener listening the sound changes his/her angle to the sound wave emitting axis of the ultrasonic transducer, the related-art apparatus cannot provide the optimum sound quality for the listener.
The present invention has been made in view of the above circumstance, and an object of the invention is to provide a method for controlling output from an ultrasonic speaker and an ultrasonic speaker system capable of constantly obtaining the optimum sound quality even when a listener changes his/her angle formed between a sound wave emitting axis of an ultrasonic transducer and a front direction axis of the listener with respect to the sound wave emitting axis.